Modified Coal-Tar Pitch

Coal-tar is usually obtained as a by-product of the distillation of coal in coke ovens, e.g. in the manufacture of metallurgical coke. It comprises a mixture of a number of hydrocarbon compounds which can be separated by distillation of the tar. Coal-tar pitch, which is the residue in the still following such a distillation is a viscous material which softens and flows when heated. Pitches are conventionally classified according to their softening points and their solubility in various solvents e.g. quinoline, benzene and dimethylformamide. The latter characteristic is usually indicated by the proportion of the pitch which is insoluble in the solvent e.g. the proportion of the pitch which is quinoline-insoluble (Q.I.) is a widely-used pitch characteristic, and is usually in North America of the order of 12 to 18%. Pitches are also obtained from petroleum but these are different in structures and characteristics especially in degree of aromaticity.

Much of the Q.I. formation takes place at the high temperature to which the tar is subjected during coke formation, and stays in the pitch residue following distillation. The presence of the Q.I. in the tar or pitch can be tolerated for a limited number of applications, however, in most of the uses, the presence of significant amounts of Q.I. is viewed as an undesirable characteristic of the material. The particulate nature of the Q.I. renders difficult virtually any process which requires the passage of the pitch through small orifices; e.g. penetration of pitch is rendered difficult. Additionally, should the Q.I have a high content of ash-forming impurities, the combustion rate of the carbon body produced by coking (or otherwise carbonizing) the pitch will be significantly increased.

Accordingly, it is an object of this invention to provide a process for decreasing and modifying the quinoline-insoluble content in coal-tar material.

It is also an object of this invention to provide a process for obtaining pitch material with a diminished content of quinoline-insoluble material.

In one aspect, the present invention is directed to specialty pitches having low Q.I., lower viscosity, lower average Q.I. particle size, lower ash content, higher toluene insoluble and beta-resin content. They may be prepared by mixing the coal-tar material with a solvent which contains at least one of n-methyl-2-pyrrolidone and the fraction of a coal-tar distillate which boils between 100.degree. and 350.degree. C., thereby to form a solvent-dissolved fraction and a solvent-undissolved fraction of the coal-tar material, separating the two fractions, where the Q.I. of the coal-tar material is concentrated in the latter fraction; and the solvent-dissolved fraction comprises coal-tar material with a decreased and modified Q.I. of the coal-tar material is concentrated in the latter fraction; and the solvent-dissolved fraction comprises coal-tar material with a decreased and modified Q.I. followed by distillation of the aforesaid solvent-dissolved fraction where the pitch has a reduced and modified Q.I and special structure and characteristics, as will be discussed further hereinbelow.

FIG. 1 is a schematic flow diagram which illustrates the steps which an embodiment of this invention entails.

FIG. 2 (FIGS. 2(a) and 2(b)) are scanning electron microscope photographs of the insoluble particulate matter in the tar and pitch respectively.

Accordingly, the present invention relates to a product obtained by a process whereby the content of quinoline-insolubles (Q.I.) in coal-tar material can be decreased and is generally less than 5%. The coal-tar material which has been thus treated can be distilled in order to obtain a pitch having a diminished Q.I., and a diminished ash content of less than 0.1% and various useful characteristics as will be discussed hereinbelow. Additionally, the Q.I. contained in the pitch so obtained will usually have a size distribution different from a conventionally obtained pitch having the same Q.I. This latter attribute of the present invention is referred to as the "modification" of the Q.I. The present invention additionally relates to a novel coal-tar pitch material having a significantly reduced viscosity for instance of the order of 35 centipoises, a softening point below 115.degree. C. and a lower average particle size, high toluene insoluble content of over 20%, and high beta-resin content of over 15%.

Broadly, the process of the present invention comprises a solvent extraction of coal-tar material in order to separate the quinoline-soluble from the quinoline-insoluble components; where the solvent chosen for this procedure contains at least one of n-methyl-2-pyrrolidone and the fraction of the coal-tar distillate which boils between 100.degree. and 350.degree. C., preferably between 200.degree. and 350.degree. C. In the following, the latter solvent will be referred to as wash oil, a term which is widely used in the art. The mixing of coal-tar material with the solvent results in the formation of two fractions which can be subsequently separated, viz. the solvent-dissolved and that undissolved in the solvent, with most of the Q.I. being localized in the latter fraction, while the former fraction comprising the coal-tar material has a greatly reduced Q.I. These fractions may be separated by conventional techniques such as filtration, centrifugation, decantation and the like. The solvent-dissolved fraction of the coal-tar material so treated can be distilled to separate the various fractions with different boiling points and obtain a pitch material with a significantly diminished Q.I. The pitch material, thus produced has a number of characteristics and uses that are associated with a low Q.I., a substantially reduced viscosity, and a relatively low average size of particulate matter. The distillation, which serves to separate the high boiling components from the pitch residue, also serves to separate the n-methyl-2-pyrrolidone from the higher boiling components with which it distils over. The n-methyl-2- pyrrolidone has a lower boiling point than the light boilers contained in the distillate and can be separated therefrom. The wash oil fraction of the distillate can be permitted to distil over with the n-methyl-2-pyrrolidone and can equivalently (individually, or in combination with the n-methyl-2-pyrrolidone) be used as a solvent in this solvent extraction process. This will permit some flexibility in the distillation procedure. The n-methyl-2-pyrrolidone and/or the wash oil fraction which will be contained in the distillate can then be recycled for use in the extraction of more coal-tar material.

The amount of solvent employed for this extraction will be partially determined by the viscosity of the resultant coal-tar-solvent mixture required for the separation technique (and which is also dependent upon the temperature of this process) and by the amount required to dissolve substantially all the quinoline soluble material. Typically, the proportion of the solvent in the mixture will usually range from about 20 to 80% of the mixture, i.e. the solvent: tar ratio will be in the range 1:4 to 4:1.

Turning now to the single FIGURE (FIG. 1), we note that n-methyl-2-pyrrolidone which is introduced via line 01, can be mixed with recycled solvent which can additionally contain some light boilers and is introduced to the mixing tank 10, via line 02 where it is mixed with the untreated tar which has been introduced to the tank via line 04, at a temperature high enough to facilitate its handling and further processing. The mixture then passes via line 06 to the separation step 20 where the solvent dissolved fraction and the fraction insoluble in the solvent are separated, possibly by differences in specific gravity or by filtration, as a result of which the former fraction leaves this step via line 08 and the latter fraction via line 07. The former fraction which contains a tar with a depleted Q.I. can be distilled in the distillation step 30 from which the n-methyl-2-pyrrolidone and the light boilers come over as the earlier fractions of the distillate and can be separated from each other, at separation step 40, if so desired or recycled via lines 13 and 15 for use as a solvent in the extraction of subsequent batches of coal-tar material. The pitch residue obtained from this distillation via line 09 will have a substantially reduced viscosity, a lower average particle size of insolubles, in addition to a reduced Q.I. content.

The advantages accruing to the user of this process are many, not the least of which is the extent of the Q.I. removal from the coal-tar which results from the use of this process. Some of the other advantages of this process stem from the solvent system used in this process viz. the n-methyl-2-pyrrolidone and/or the light boiling cut of the tar distillate; in particular, the ease with which the solvent can be used, recovered, reused; and the facility with which these operations can be integrated into a conventional pitch production process.

A surprising outcome of this process is the manner in which the viscosity of the pitch and the size distribution of the Q.I. is affected. The size distribution of a pitch derived from an unextracted tar, or one extracted with a solvent different from that of the present invention, is often characterized by a greater degree of particulate agglomeration. The pitch material thus obtained is characterized by a viscosity lower than that of pitches obtained by the distillation of the untreated coal tar. Also, the pitch obtained has uniquely high toluene insoluble content, beta-resin content and low ash content. Such a pitch cannot be obtained by the conventional way involving purifying the pitch. As a tentative explanation, it would appear that during distillation of unpurified pitch, various side reactions occur inhibiting the production of such a unique pitch. These are particularly important where the particule size of the pitch and other characteristics are important e.g. in applications such as the impregnation of prebaked electrodes.

The following examples will serve to illustrate the invention.

This example illustrates how the Q.I. of a conventional coal-tar material can be decreased by the present invention using the light boilers of coal-tar distillation (wash oil) as the solvent.

A sample of coal-tar having a Q.I. of 6.5%, and representative of the output obtained from steel mills was mixed with wash oil in a tar to wash oil ratio of 3:2. The mixture was filtered using a Buchner funnel and No. 1 analytical filter paper with a moderate vacuum being applied to accelerate the process. The Q.I. of the treated tar was measured. The filtrate was then distilled under atmospheric pressure, and the Q.I. content of the pitch residue determined in accordance with ASTM D2318. The experimental results are summarized in Table 1 below. The above procedure was repeated with two other commercially available coal-tar samples having Q.I. values of 7.0 and 23.7% respectively. In each of the cases, the Q.I. in the treated tar was a small fraction of that in the original sample.

A possibly more remarkable outcome which was indicative of the degree to which the process resulted in the Q.I. removal was the low Q.I. content in the pitch residue as compared to the original tar. This is surprising because Q.I. is normally concentrated in the pitch, partly due to a large decrease in the volume, and partly due to some thermal cracking which occurs during the distillation. Nonetheless, in all the cases the Q.I. in the pitch was a small fraction of that in the tar prior to filtration.

The above procedure was repeated with the same three tar samples, but with a tar-solvent ratio of 1:4. The experimental results which are qualitatively substantially the same as the preceding set; are also summarized in Table 1 below.

This example illustrates the use of n-methyl-2-pyrrolidone as the solvent in the extraction of the Q.I. from the tar.

Tar samples identical to those in Example 1 were subjected to essentially the same procedure and tests as described above in the preceding Example. The results of these experimental runs which are summarized in Table 2 below follow essentially the same pattern as in the preceding Example. The treated tar has a greatly reduced Q.I. which is also manifested in the low Q.I. content of the pitch residue remaining after the distillation of the tar.

This Example presents a qualitative comparison of particle sizes of insolubles in tar which has been extracted using a conventional solvent (in this case quinoline), and that which is residual in pitch after an extraction of the precursor tar according to the present invention.

FIG. 2(a) is a scanning electron microscope (S.E.M.) photograph of residual tar Q.I. following an extraction using quinoline. FIG. 2(b) is a S.E.M. photograph of the insolubles in a pitch prepared from an identical tar after extraction with n-methyl-2-pyrrolidone.

It will readily be seen that typical particle sizes in the former photograph are greater than about 1 micrometer, with sizes of the agglomerated particles ranging up to about 4 micrometer. The particle sizes in the latter photograph (FIG. 2(b)) range from less than about 0.1 micronmeter to about 0.5 micronmeter, with the larger agglomerates being about 1 micrometer in size.

The difference in the particle size distribution is all the more substantial when we note that the particles in latter photograph include not only the insolubles following the extraction of the tar, but additionally include the "secondary" Q.I. produced during the tar distillation to produce the pitch, where the original Q.I. is concentrated and function as nuclei for the growth of Q.I. particles.

This example illustrates the unique properties of the tar obtained by the applicant's process as compared against those obtained in accordance with the prior art in North America prior to applicant's invention, as well as in Europe where the raw material differs.

The properties are shown on page 12.

As can be easily seen, applicant's product is unique, and has particular advantages, for instance as far as the high toluene insoluble content and beta-resin are concerned, which are important for instance inter-alia for the making of impregnating pitch in the electrode field.

The above described procedure can be subjected to a great variety of modifications which will be evident to those skilled in the art and which fall within the scope of the appended claims.